Dc-ac converter

ABSTRACT

The invention relates to a DC-AC converter using a transistor whose base-emitter circuit is shunted by a transformer winding. According to the invention, a total current of comparatively high value is generated in this winding due to induction and conduction so that this current removes charge carriers from the base of the transistor when this transistor becomes nonconducting. As a result this transistor is quickly ready again for blocking a voltage across its main electrodes. The invention is especially important in the use of highly resistive transistors for a high voltage.

O Umted States Patent 1151 3,662,249 Wijsboom 1 51 May 9, 1972 s41 DC-ACCONVERTER 3,350,661 10/1967 316cm et a1 ..321/45 R 3,081,437 3/1963Radcliffe, Jr... ...321/451 UX [721 Emmasmgd- 3,256,495 6/1966 Hunter..331/113.1 3,020,491 2/1962 Kurtz ..321/45 s 73 Assignee; s phflips NewYork, y 3,051,914 8/1962 Brown ..331/1 13.1

1 Filed! l- 1970 Primary Evaminer-William H. Beha. Jr. [211 App No; 68401 Attorney-Frank R. Trifari [57] ABSTRACT [30] Foreign Application Theinvention relates to a DC-AC converter using a transistor Sept. 18. 1969Netherlands ..69l4125 o e base-emitter circuit is shunted y atransformer winding. According to the invention, a total current ofcompara- 52 us. c1 "321/11, 321/45 R, 331/1 13 A lively high value isgenerated in this winding due to induction [51] Int. Cl. I andconduction so that this current removes charge carriers 58 Field ofSearch ..321/4s,4ss;331/113.1 the base f the transistor when this"ansislor becomes I non-conducting.-As a result this transistor isquickly ready [56 Rdmnm Cited again for blocking a voltage across itsmain electrodes. The invention is especially important in the use ofhighly resistive UNITED STATES PATENTS transistors for a high voltage.

3,134,948 5/1964 wilting ..331/1 13.1 9Claims, ZDrawlng FiguresPATENTEDMAY 9 I972 SHEET 1 OF 2 T /LI Fig.1

1N VENTOR.

DAN B. WIJSBOOM AGENT DC-AC CONVERTER This invention relates to a DC-ACconverter provided with a transistor and a transfonner in which awindingof said transfonner connects the base and the emitter of said transistortogether and in which a circuit generating control signals is coupled tosaid transformer winding.

A known DC-AC converter of the kind mentioned above is described, forexample, in U.S. Pat. No. 3,084,283. An advantage of the circuitdescribed in this U.S. patent is that after a transistor has becomenon-conducting, the remaining charge carriers in the base of thistransistor are removed by means of a voltage induced in the transformerwinding so that this transistor is soon ready again to cut off a voltageacross its main electrodes. A drawback of the known circuit is, however,that a second transistor in the circuit is rendered conductingsubstantially simultaneously with the cutoff of the first transistor inthe circuit. This might give rise to serious damage to the converter ifthe transistors are allowed to conduct simultaneously for a short timeperiod.

It is an object of the present invention to provide a DC-AC converter inwhich the charge carriers are rapidly removed from the base region aftera transistor has been turned off? and in which a second transistorcannot conduct simultaneously with the first transistor.

According to the invention a DC-AC converter is provided with atransistor and a transformer in which a winding of said transformerconnects the base and the emitter of said transistor together and inwhich a circuit generating control signals is coupled to saidtransformer winding. A feature of the invention is that the two ends ofsaid transformer winding are in electrically conducting connection withthe circuit generating the control signals and that the input terminalsof the converter are connected by means of a series arrangement whichincludes at least a different winding of the transformer and the mainelectrodes of the transistor. The winding sense of the last mentionedtransformer winding is chosen so that a current is induced in thefirst-mentioned transformer winding during the conducting period of thetransistor. The latter current flows in the same direction as a currentwhich is generated by the circuit generating control signals in thefirst-mentioned transformer winding so that the said currents togetherremove charge carriers from the base of the transistor through thefirst-mentioned transformer winding when the transistor is cut-off.

The invention is based on the recognition of the fact that a current ofcomparativelygreat intensity is generated during the conducting periodof the transistor in an inductor which is arranged between the base andthe emitter of said transistor, which current is maintained when thetransistor becomes nonconducting and thereby removes charge carriersfrom the base of the transistor. According to the invention this largecurrent consists of two components, namely one component which isobtained from the circuit generating control signals and a secondcomponent which is generated through a further transformer winding whichis present in the main circuit, that is to say, in the circuit includingthe main electrodes of the transistor. These two components combinedproduce a comparatively large current in the said inductor between thebase and the emitter of the transistor.

' A DC-AC converter according to the invention may be equipped with, forexample, low-voltage transistors.

A DC-AC converter according to the invention is preferably equipped witha high-voltage transistor while the operating frequency of the converteris more than 18,000 Hz. in this connection a high-voltage transistor isunderstood to mean a transistor having a cut-off voltage of at least 300volts. Such a transistor is made, for example, of highly resistivesilicon.

An advantage of this preferred embodiment is that transistors may beused which can withstand a comparatively large voltage but at the sametime do not require a fairly low frequency so as to avoid switchinglosses. On the contrary, due to the rapid removal of the charge carriersby the dual current, this preferred embodiment makes it possible toobtain a converter which is suitable for both a high direet voltage andwhich may operate at a high frequency.

In order that the invention may be readily understood and carried intoeffect, a few embodiments thereof will now be described in detail by wayof example with reference to the accompanying diagrammatic drawings, inwhich:

FIG. 1 shows an electric circuit of an arrangement according to theinvention, and

FIG. 2 shows a few graphs illustrating diagrammatically the current asa'function of time in different parts of the circuit arrangement of FIG.1.

in FIG. 1 the reference numerals l and 2 denote connecting terminalswhich are intended to be connected to a direct voltage source. Thereference numeral 1 denotes the positive terminal and the referencenumeral 2 denotes the negative terminal. The terminals 1 and 2 areconnected together through two series arrangements. One seriesarrangement includes 'a' winding 3 of a transformer 4, a winding 5 of atransformer 6, v

the main electrodes of a transistor 7, a tapping 8, a diode 9 and a fuse10. The diode 9 provides a safeguard against a wrong connection of theconnecting terminals 1 and 2 to the direct current source. The otherseries arrangement includes a winding 11 of the transformer 4, a winding12 of a transformer 13, the main electrodes of a transistor 14, thetapping 8, the diode 9 and the fuse 10. These two series arrangementsconstitute the main current series arrangements of the converter. Inad-' dition the circuit arrangement includes a circuit for generatingcontrol signals. This circuit includes a winding 15 of the transformer4, a diode l6, and a capacitor 17 which is shunted by a resistor 18. Aconnecting wire 19, which connects the winding 15 to a connectionbetween the tapping 8 and the diode 9, is likewise associated with thecircuit generating control signals. A further section of the circuitgenerating control signals is substantially equal to the first sectionof this circuit and includes a winding 20 of the transformer 4, a diode21 and a capacitor 22 which is shunted by a resistor 23. The wire 19 iscommon for the two sections of the circuit generating control signals.

Furthermore, the circuit arrangement includes a winding 24 of thetransformer 6. This winding 24 connects the base and the emitter of thetransistor 7 together. Furthermore, the ends of this winding 24 areelectrically connected to the circuit generating control signals 15 to23 inclusive).

. Similarly the base and the emitter of the transistor- 14 are connectedtogether by a secondary winding 25 of the transformer 13. Also the endsof this winding 25 are in electrically conducting connection with thefurther section of the circuit generating control signals.

The circuit diagram further shows that the base-emitter circuit of thetransistor 7 is shunted by a series arrangement of a diode 26 and aresistor 27 and that the base-emitter circuit of the transistor 14 isshunted by a series arrangement of a diode 28 and a resistor 29.Furthermore, a capacitor 30 is present which connects the two extremeends of the windings 3 and 11 of the transformer 4 together.Furthermore, an auxiliary circuit for starting the converter is present.This auxiliary circuit includes a resistor 31 which is connected to theinput terminal 1 and a diac 32 which is connected in series with theresistor 31. The other end of this diac 32 is connected to theconnection between the resistor 18 and the winding 24. Furthermore, aconnection between the resistor 31 and the diode 9 is present. Thisconnection includes a capacitor 33. A diode 34 is connected to thejunction between the resistor 31 and the diac 32. The other end of thisdiode 34 is connected to the co|- lector of the transistor 7.

Transformer 4 has a secondary winding which is denoted by the referencenumeral 35. Two series-arranged low-pressure mercury vapor dischargelamps 36 and 37 are connected to this winding 35. The lamp 36 isprovided with two preheated electrodes 38 and 39. The lamp 37 isprovided with two preheated electrodes 40 and 41. The preheatedelectrode 38 is fed from a section 42 of the winding 35. Theseries-arranged windings 39 and 40 are connected to an additionalwinding 43 of the transformer 4. The preheated electrode 41 of the lamp37 is connected in a manner corresponding to the electrode 38, namely toa section 44 of the winding 35.

The circuit arrangement described operates as follows. When theterminals 1 and 2 are connected to a direct voltage source, a currentwill first flow from terminal 1 via the resistor 31 to the capacitor 33and via the diode 9 back to the terminal 2. When this capacitor 33 issufficiently charged, the diac 32 will break down and thereby render thebase of the transistor 7 positive relative to its emitter so that thistransistor 7 begins to conduct. Subsequently a current will start toflow through the first main series arrangement, to wit from terminal 1through winding 3, winding 5, the main electrodes of transistor 7,tapping 8, diode 9, and fuse 10 to terminal 2. This current will inducea voltage in the winding 11 in such a manner that the side of thewinding 11 remote from the winding 3 receives a voltage which ispositive relative to the junction of the windings 3 and 11. As a resultof the joint voltages across the windings 3 and 11 the capacitor 30 willnow be charged.

However, due to conduction in the transistor 7, the capacitor 33 will bedischarged, namely through the diode 34 and the main electrode circuitof the transistor 7. As a result this transistor will be cut-off again,because no base current is available anymore to maintain this transistorconducting. As a result the capacitor 30 will be discharged across thewindings 11 and 3. This discharge induces, inter alia, a voltage in thewindings 20 and 15. These windings are wound in such a manner that theend of the winding 20 which is remote from the winding 15 now receives apositive voltage relative to the junction of the windings and 15. As aresult a current will start to flow through the diode 21 and thecapacitor 22. This current is then split up into a portion which flowsthrough the base-emitter junction of the transistor 14 and a portionwhich flows through the winding of the transformer 13. These twocurrents then flow jointly through the return line 19 to the winding 20.The portion of the current flowing through the base of the transistor 14renders this transistor conducting. In a corresponding manner to thatdescribed with reference to the first main series arrangement, a maincurrent will start to flow from terminal 1 through winding 11, throughwinding 12, the main electrodes of the transistor 14, the tapping 8, thediode 9, and the fuse 10 to the terminal 2 when the transistor 14conducts. When the capacitor 22 is charged as a result of the currentflowing through the diode 21, no further current will flow through thiscircuit and hence the transistor 14 will become non-conducting. Due tothe current which initially flowed through the winding 25, and due to anadditional current in the winding 25 induced therein by winding 12 andflowing in the same direction as the initial current, a total currentnow flows through said inductor 25, which current removes chargecarriers from the base of the transistor 14 when the transistor 14becomes non-conducting. The advantage thereof is that this transistor isthen quickly brought to a state where it is again ready to block avoltage across its main electrodes. During the conducting state oftransistor 14, the capacitor was charged in a reversed sense. Since thetransistor 14 has become non-conducting again, the capacitor 30 isdischarged again, but now across the windings 3 and 11 which leadscauses to a voltage to be induced in the winding 15 of the transformer4. The voltage across the winding 15 produces, through the diode 16, thecapacitor 17, etc., a conducting state in the transistor 7. Subsequentlythe described procedure is repeated, provided that components of currentare now generated in the winding 24 of the transformer 6. This dualcurrent leads to the fast removal of charge carriers from the base ofthe transistor 7 when this transistor 7 is once again cut-off.

The currents flowing in the windings 3 and 11 are magneticallytransferred to the windings 35, 42, 43 and 44 on the load side of theconverter. The two lamps 36 and 37 are then energized. Since thetransformer 4 is formed as a leakage transformer, the discharge of theselamps is stabilized by means of this transformer.

FIG. 2 shows a few current vs. time graphs. These graphs illustrate asomewhat diagrammatical view of the current variation.

The current (i flowing through the diode 16 is plotted as a function oftime t on the first line of FIG. 2.

The collector currents (i of the transistor 7 is plotted as a functionof time on the second line of FIG. 2.

The current 13 (3) which is induced in the winding 24 by the currentflowing through the winding 5 is plotted on the third line of FIG. 2.

The current i, ,(S) in the winding 24 which is generated therein up toan instant t by the circuit generating control signals is plotted on thefourth line of FIG. 2. The reference 1, represents the instant at whichthe transistor 7 becomes nonconducting. After this instant t, adecreasing current will flow through the winding 24 as a result of theinductive character of this winding in combination with the presence ofresidual charge carriers in the base of transistor 7.

The total current 1T flowing through the winding 24 is plotted as afunction of time on the fifth line of FIG. 2 (thus the combination ofthe two previous lines).

The base current i of the transistor 7 is plotted as a function of timeon the last line of FIG. 2.

The first line of FIG. 2 shows that the current flowing through thediode 16 has a decreasing variation. The collector current of thetransistor 7 has an increasing variation up to the instant t due to theinfluence of, for example, the winding 3.

The penultimate line of FIG. 2 shows that a comparatively large currentflows in the winding 24 at the instant The base current of thetransistor 7, as shown on the last line of FIG. 2, consists of thecurrent flowing through the diode 16 minus the total current flowingthrough the inductor 24. It is evident that this base current, as isshown in FIG. 2, is greatly negative at the instant The downward peak onthis last line of FIG. 2 shows the removal of the charge carriers fromthe base of the transistor 7. It is evident thatcorresponding graphs maybe given for the transistor 14 and the inductor 25.

The diode 26 and 28 (see FIG. I) serve as a safeguard in combinationwith the resistors 27 and 29, respectively.

In one embodiment the battery voltage was volts and the transistors werehigh-voltage silicon transistors. The inductance of each of the windings24 and 25 was approximately 100 H. In that case each of the lamps 36 and37 were lowpressure mercury vapor discharge lamps of 20 watts having anoperating voltage of approximately 50 volts. The operating frequency ofthe converter was approximately 20,000 Hz.

What is claimed is:

l. A DC-AC converter comprising a pair of input terminals adapted forconnection to a source of voltage, a transistor, a transformer having awinding which connects the base and the emitter of said transistortogether, a circuit for generating control signals coupled to saidtransistor base electrode, means coupling the two ends of saidtransformer winding in electrically conducting connection with thecircuit for generating control signals to generate a first currentcomponent in said winding, means connecting a series arrangement whichincludes a second winding of the transformer and the main electrodes ofthe transistor across the input terminals of the converter, the windingsense of the second transformer winding being arranged in a manner suchthat a second current component is induced in the first-mentionedtransformer winding during the conducting period of the transistor,which second current component flows in the same direction as said firstcurrent component in the first-mentioned transformer winding so that thesaid first and second currents together remove charge carriers from thetransistor base through the first-m entioned transformer winding whenthe transistor becomes nonconducting.

2. A DC-AC converter as claimed in claim 1 in which the transistor is ahigh-voltage transistor, characterized in that the operating frequencyof the converter is more than 18,000 Hz.

3. A DC-AC converter comprising a pair of input terminals adapted forconnection to a source of voltage, a transistor having base, emitter andcollector electrodes, a transformer having first and second windings,means connecting said first winding between the base and emitterelectrodes of said transistor, a circuit for generating control signalscoupled to said base electrode to cause said transistor to repetitivelyturn on and off, means electrically connecting said first winding tosaid signal generating circuit to cause a first current component toflow in said winding, means connecting said second winding in serieswith the transistor emitter-collector electrodes across said inputterminals thereby to induce a second current component in the firstwinding due to current flow in the second winding during the transistorconduction period, said first and second windings being relatively woundso that said second current component flows in the same direction assaid first current component in said first winding whereby said firstand second currents are combined to rapidly sweep charge carriers fromthe transistor base region via the first winding when the transistor iscut-off.

4. A converter as claimed in claim 3 wherein said circuit for generatingcontrol signals comprises a second transformer with a first windingconnected in series with the emitter-collector electrodes of saidtransistor and the second winding of the first transformer across theconverter input terminals and a second winding coupled across thebase-emitter circuit of the transistor to derive said first currentcomponent in said first winding of the first transformer.

5. A converter as claimed in claim 4 wherein said circuit for generatingcontrol signals further comprises a unidirectional current elementconnected in series with the second winding of said second transformerand with the same polarity as the base-emitter junction of thetransistor.

6. A converter as claimed in claim 4 further comprising a capacitorcoupled to the first winding of said second transfonner to formtherewith a self-oscillating resonant circuit.

7. A converter as claimed in claim 3 further comprising an auxiliarystarting circuit that includes a resistor and capacitor coupled to saidinput terminals and a voltage breakdown element coupled between the baseof the transistor and the junction point of said resistor and capacitor.

8. A converter as claimed in claim 7 further comprising a diodeconnected between said junction point and the collector electrode of thetransistor and poled so as to discharge the capacitor through saidtransistor when the transistor is triggered into conduction.

9. A converter as claimed in claim 3 further comprising a secondtransistor connected in a push-pull circuit arrangement with the firsttransistor across said input terminals, a second transformer having athird winding connected between the base and emitter of the secondtransistor and a fourth winding connected in series with the secondtransistor emittercollector electrodes across said input terminals, asecond circuit for generating control signals coupled to said secondtransistor base electrode to cause the second transistor to repetitivelyturn 011' when said first transistor turns on and vice versa, meanselectrically connecting said third winding to said second signalgenerating circuit to cause a first current component to flow in saidthird winding, said third and fourth windings being relatively wound sothat a second current component is induced in the third winding by thecurrent flow in the fourth winding during the conduction period of thesecond transistor and in the same direction as said first currentcomponent whereby said first and second currents are combined to rapidlysweep charge carriers from the second transistor base region via thethird winding when the second transistor is cut-off.

I i l I

1. A DC-AC converter comprising a pair of input terminals adapted forconnection to a source of voltage, a transistor, a transformer having awinding which connects the base and the emitter of said transistortogether, a circuit for generating control signals coupled to saidtransistor base electrode, means coupling the two ends of saidtransformer winding in electrically conducting connection with thecircuit for generating control signals to generate a first currentcomponent in said winding, means connecting a series arrangement whichincludes a second winding of the transformer and the main electrodes ofthe transistor across the input terminals of the converter, the windingsense of the second transformer winding being arranged in a manner suchthat a second current component is induced in the first-mentionedtransformer winding during the conducting period of the transistor,which second current component flows in the same direction as said firstcurrent component in the firstmentioned transformer winding so that thesaid first and second currents together remove charge carriers from thetransistor base through the first-mentioned transformer winding when thetransistor becomes non-conducting.
 2. A DC-AC converter as claimed inclaim 1 in which the transistor is a high-voltage transistor,characterized in that the operating frequency of the converter is morethan 18,000 Hz.
 3. A DC-AC converter comprising a pair of inputterminals adapted for connection to a source of voltage, a transistorhaving base, emitter and collector electrodes, a transformer havingfirst and second windings, means connecting said first winding betweenthe base and emitter electrodes of said transistor, a circuit forgenerating control signals coupled to said base electrode to cause saidtransistor to repetitively turn on and off, means electricallyconnecting said first winding to said signal generating circuit to causea first current component to flow in said winding, means connecting saidsecond winding in series with the transistor emitter-collectorelectrodes across said input terminals thereby to induce a secondcurrent component in the first winding due to current flow in the secondwinding during the transistor conduction period, said first and secondwindings being relatively wound so that said second current componentflows in the same direction as said first current component in saidfirst winding whereby said first and second currents are combined torapidly sweep charge carriers from the transistor base region via thefirst winding when the transistor is cut-off.
 4. A converter as claimedin claim 3 wherein said circuit for generating control signals comprisesa second transformer with a first winding connected in series with theemitter-collector electrodes of said transistor and the second windingof the first transformer across the converter input terminals and asecond winding coupled across the base-emitter circuit of the trAnsistorto derive said first current component in said first winding of thefirst transformer.
 5. A converter as claimed in claim 4 wherein saidcircuit for generating control signals further comprises aunidirectional current element connected in series with the secondwinding of said second transformer and with the same polarity as thebase-emitter junction of the transistor.
 6. A converter as claimed inclaim 4 further comprising a capacitor coupled to the first winding ofsaid second transformer to form therewith a self-oscillating resonantcircuit.
 7. A converter as claimed in claim 3 further comprising anauxiliary starting circuit that includes a resistor and capacitorcoupled to said input terminals and a voltage breakdown element coupledbetween the base of the transistor and the junction point of saidresistor and capacitor.
 8. A converter as claimed in claim 7 furthercomprising a diode connected between said junction point and thecollector electrode of the transistor and poled so as to discharge thecapacitor through said transistor when the transistor is triggered intoconduction.
 9. A converter as claimed in claim 3 further comprising asecond transistor connected in a push-pull circuit arrangement with thefirst transistor across said input terminals, a second transformerhaving a third winding connected between the base and emitter of thesecond transistor and a fourth winding connected in series with thesecond transistor emitter-collector electrodes across said inputterminals, a second circuit for generating control signals coupled tosaid second transistor base electrode to cause the second transistor torepetitively turn off when said first transistor turns on and viceversa, means electrically connecting said third winding to said secondsignal generating circuit to cause a first current component to flow insaid third winding, said third and fourth windings being relativelywound so that a second current component is induced in the third windingby the current flow in the fourth winding during the conduction periodof the second transistor and in the same direction as said first currentcomponent whereby said first and second currents are combined to rapidlysweep charge carriers from the second transistor base region via thethird winding when the second transistor is cut-off.